A tumor can also grow as result of signals from inside the tumor cell. 

About 20% of all breast cancers, among the most aggressive ones, show an abnormally high number of HER2 gene copies. 

This gene codifies the production of HER2 proteins that sit on the external surface of the cells stimulating their division.

A high number of HER2 in a tumor means that its cells divide quicker than a normal cell.

In this type of cancers (HER2+), the tumor growth is controlled using directed targeted therapies at the HER proteins. An example is the drug trastuzumab(Herceptin®) or similar. 

In the tumor pathology report, one of the tests done is to determine the number of HER2 copies of the tumor. 

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